Sequences

A sequence is a stored object that simply generates a sequence of numbers in a monotonically ascending (or descending) order. Sequences provide a way to have the database manager automatically generate unique integer and decimal primary keys, and to coordinate keys across multiple rows and tables. A sequence can be used to exploit parallelization, instead of programmatically generating unique numbers by locking the most recently used value and then incrementing it.

Sequences are ideally suited to the task of generating unique key values. One sequence can be used for many tables, or a separate sequence can be created for each table requiring generated keys. A sequence has the following properties:

• Can have guaranteed, unique values, assuming that the sequence is not reset and does not allow the values to cycle.
• Can have increasing or decreasing values within a defined range.
• Can have an increment value other than 1 between consecutive values (the default is 1).
• Is recoverable.

Values for a given sequence are automatically generated by the database manager. Use of a sequence in the database avoids the performance bottleneck that results when an application implements sequences outside the database. The counter for the sequence is incremented (or decremented) independently from the transaction.

In some cases, gaps can be introduced in a sequence. A gap can occur when a given transaction increments a sequence two times. The transaction may see a gap in the two numbers that are generated because there may be other transactions concurrently incrementing the same sequence. A user may not realize that other users are drawing from the same sequence. Furthermore, it is possible that a given sequence can appear to have generated gaps in the numbers, because a transaction that may have generated a sequence number may have rolled back. Updating a sequence is not part of a transaction's unit of recovery.

A sequence is created with a CREATE SEQUENCE statement. A sequence can be referenced using a sequence-reference. A sequence reference can appear most places that an expression can appear. A sequence reference can specify whether the value to be returned is a newly generated value, or the previously generated value. For more information, see CREATE SEQUENCE and Sequence reference.

Although there are similarities, a sequence is different than an identity column. A sequence is an object, whereas an identity column is a part of a table. A sequence can be used with multiple tables, but an identity column is part of a single table.